This read-only 64-bit counter tracks the total number of bytes read from this disk device since the host last booted, functioning as the wrap-resistant counterpart to the 32-bit diskIONRead. Admins on high-throughput systems prefer this counter over diskIONRead since a busy disk can wrap the 32-bit version relatively quickly, making the 64-bit version more reliable for accurate throughput calculation over longer polling intervals. On a disk pushing gigabytes of read traffic per hour, an admin would poll diskIONReadX rather than diskIONRead to avoid counter-wrap errors in their throughput calculations.